1. Field of Invention
This invention relates to impeller-driven watercraft having a concavity formed in the hull of the watercraft for accommodating the impeller so that the impeller operates at high efficiency.
2. Description of the Prior Art
A conventional watercraft equipped with an inboard engine has a downwardly-inclined elongate drive shaft that interconnects the output shaft of the engine and a propeller. A water-tight opening is formed in the hull so that the elongate drive shaft can extend therethrough. A support strut depends from the bottom of the hull and a housing at the lower end of the strut receives the elongate drive shaft. The propeller is positioned on the trailing side of the housing, at the trailing end of the elongate drive shaft. An inboard or outboard-mounted rudder trails the propeller.
This well-known arrangement of parts results in a low efficiency watercraft because most of the elongate drive shaft and all of the other parts, i.e., the strut, the housing, the propeller and the rudder are all mounted below the hull of the watercraft, thereby creating drag. The propeller and other exposed parts are also subject to damage by submerged rocks and the like.
Inventors have therefore improved the conventional design by forming a tunnel in the hull, at the stern end thereof, in the form of a concavity, and positioning the aforesaid parts in the tunnel. This enables the elongate drive shaft to be positioned in a horizontal plane or nearly in a horizontal plane. An example of such a watercraft is disclosed in co-pending U.S. patent application Ser. No. 10/855,569 to the present inventor. Further examples are disclosed in U.S. Pat. No. 3,659,547 to Stuart and in U.K. patent application No. 2,248,433 to Renato Levi Limited. This arrangement of parts reduces drag so that the engine operates at higher efficiency. The parts in the tunnel are also protected from damage by submerged objects. A twin engine watercraft is provided with two tunnels, equidistantly spaced on opposite sides of the longitudinal axis of symmetry of the watercraft.
An intake grate extends over the mouth of each tunnel to admit water but not debris into the concavity.
The use of a stator in conjunction with a propeller also increases the efficiency of the propeller. A propeller, when encircled by a housing having a diameter only slightly greater than the diameter of the propeller, becomes an impeller by definition.
Although watercraft having at least one tunnel formed in the hull operate at higher levels of efficiency than watercraft lacking such tunnel or tunnels, there remain a few areas where such watercraft could be improved.
For example, at high speeds air can be entrained into the tunnel. Specifically, air flows along the top of the tunnel so that the tunnel is not full of water. The air causes turbulence within the tunnel as it mixes with water as it flows through the impeller. A laminar flow through an impeller is more desirable than a turbulent flow because in a laminar flow, only water flows through the impeller. Thrust is lost when air flows through an impeller.
Thus there is a need for an improved tunnel design that inhibits the formation of airflow through the tunnel. Such an improved design would produce a high efficiency laminar flow through the impeller.
Tunnel designs are also known where a nozzle has a diameter that is only about half the diameter of the impeller. Such reduced-diameter nozzles are commonly provided in watercraft propulsion systems that harness the power created by the action and reaction characteristics of a jet. This reduces engine efficiency by creating a substantial back pressure.
Thus there is a need for a propulsion system that does not rely upon jet propulsion characteristics so that the nozzle may have a large diameter relative to impeller diameter, thereby reducing back pressure and increasing engine efficiency.
The bow of a high-performance boat tends to lift up from the water and the stern thereof tends to enter more deeply into the water at high speeds.
There is a need, therefore, for a boat design that inhibits bow-lifting and stern-deepening.